Oxide-type honeycomb structures for application as catalyst carriers have already reached the level of practical utilization with regard to members subjected to long-term exposure to which temperatures, for example, combustion catalysts and automotive three-way catalysts, and developmental efforts targeted to additional improvements in their properties are actively moving forward. Among these oxide-type honeycomb structures, cordierite, by virtue of its high melting point of about 1400° C., its very low coefficient of thermal expansion, and its excellent resistance to thermal shock, is employed for the honeycomb structures that are used as catalyst carriers in high temperature zones where the temperature exceeds 800° C., e.g., for automotive three-way catalysts, combustion catalysts for gas turbines, and catalysts for the purification of high-temperature gases.
While the usefulness of cordierite as a catalyst carrier has already been recognized, it is quite difficult using prior-art methods for producing porous cordierite to fabricate heat-stable porous cordierite that exhibits a high specific surface area, and as a consequence current practice is to coat the inner walls of the cordierite honeycomb with, for example, gamma-alumina, which has a high specific surface area, and to support the catalyst on this coating. However, a problem with gamma-alumina is the difficulty of maintaining its high specific surface area at high temperatures of 1000° C. or above due to the transformation to alpha-alumina and the advance of sintering.
Numerous reports of research on the fabrication of high specific surface area cordierite sinters have appeared to date. However, sintering progresses in the high specific surface area porous cordierites reported to date when they are heated at high temperatures exceeding 1000° C., resulting in a dramatic decline in specific surface area, and the high specific surface area porous cordierites reported to date therefore cannot be used as catalyst carriers for members exposed to high temperatures.
The present inventors have to date succeeded in the development of porous cordierite bodies constituted of needle-shaped cordierite crystals having submicron diameters, and, as described in the references to the prior art have introduced honeycomb structures made of porous cordierite that directly utilize these porous cordierite bodies and honeycomb structures made of porous cordierite that employ the application of a coating on the interior walls of the porous cordierite body (Japanese Patent Application Laid-open Numbers 2003-321280, 2003-212672, 2003-025316, 2002-355511, 2002-119870, 2002-172329, 2001-310128, and Hei 11-171537). And as noted above, in those cases where cordierite is used in a member exposed to high temperatures, at present no method is available other than the application of a coating of, e.g., gamma-alumina, on the interior walls of the honeycomb structure.
Still, there have been problems with the application of a coating on the interior walls of the honeycomb structure, such as a reduction in specific surface area due to debonding of the coating layer and the advance of sintering as well as pressure losses by the gas stream passing through the honeycomb structure, and there has been demand for a solution to these problems. Given the preceding, there has been strong desire in the concerned technical field for the development of a high specific surface area porous cordierite body that exhibits little sintering-induced decline in specific surface area even upon long-term heating at high temperatures but in fact to date such a high temperature-stable, high specific surface area porous cordierite body has not been developed.